The invention concerns circuit arrangements for operating electrical incandescent bulbs, particularly low-volt halogen incandescent bulbs.
This type of circuit arrangement contains as principal component a self-starting current-feedback inverter in half-bridge circuit, which chops the low-frequency mains voltage or d.c. voltage of the electrical supply into a high-frequency voltage. This voltage is then transformed by means of a power transformer to the low voltage of the halogen incandescent bulbs that are used, for example, usually to 6, 12 or 24 V. Such circuit arrangements are thus called electronic transformers or electronic converters. A detailed explanation of the functioning of electronic converters is disclosed, for example, in EP Patent 264,765 and DE-OS (Unexamined) 4,011,742.
A protective device is indispensable in order to prevent self-destruction of the circuit arrangement with a possible danger of fire as well as endangering of persons when an overload or short circuit occurs, i.e., when an inadmissible low-ohm load arises, for example, caused by parallel connection of several bulbs or by short circuiting of the bulb leads. For this purpose, a detector circuit is necessary, which supplies a suitable detection signal in the case of overload or short circuit. For brevity this circuit will be called the overload detector circuit in the following. The detection signal then serves as the control signal for a cutoff circuit, which disconnects the electronic converter at least temporarily.
After disconnection, the high-frequency oscillation of the inverter must be initiated again. Commonly, electronic converters contain a starting or triggering generator, which assumes this task. The electronic converter must be separated for a short time from the supply voltage. After a repeated application of the supply voltage, the inverter again oscillates.
A self-oscillating inverter in half-bridge circuit for a low-volt lamp with a protection circuit and a current measurement member is known from DE 42 42 120 A1. The current measurement member comprises essentially four resistors and a zener diode which is connected between the source of the lower half-bridge field effect transistor and the reference potential. A disadvantage of this solution is that the current measuring member requires a relatively large number of components. Also, the signal tapped from the resistance circuit for controlling the protective circuit is approximately proportional to the lamp current. In order to be able to distinguish at all between normal case and overload case, the protective circuit must be configured to have a very steep flank.
EP 0 647,084 A1 discloses a circuit arrangement with a self-starting half-bridge transformer and a detector circuit. The detector circuit comprises a serial connection of a diode and a resistance, which is connected in parallel to the bridge capacitor connected to ground potential. In addition, the bridge capacitors and the power transformer are dimensioned in such a way that the diode is conductive only when overload occurs. The serial resistance then transforms the current flowing through the diode into a voltage signal, which serves as a detection signal and controls a cutoff circuit. A disadvantage of this solution is the fact that the dimensioning of the half-bridge transformer must be fine-tuned to the detector circuit.